Television systems which receive digital video data streams that have been encoded and compressed by an encoder, such as an MPEG video encoder, require a decoder, such as an MPEG video decoder, in order to decode and decompress the incoming digital video data streams. In order to ensure proper operation of such television systems, it is necessary, both in the factory and later in service, to test the MPEG video decoders of television systems to determine if the MPEG video decoders are properly decoding and decompressing the incoming digital video data streams.
One possible way of testing an MPEG video decoder is to supply the MPEG video decoder with a set of test frames that have been processed through an MPEG encoder, and to inspect the results of the decoding of these frames by the MPEG video decoder. This test procedure is permissible because a given MPEG video decoder, that fully decodes the entire input digital video data stream, will always produce exactly the same set of reconstructed pixel values for a known specific input digital video data stream. This characteristic of an MPEG video decoder allows correct decoder operation to be verified by sending a set of known input test frames to the decoder to be tested and by observing each output pixel in every corresponding output frame from the MPEG video decoder to be sure that the output pixel has the correct value.
In such a testing procedure, the MPEG video decoder being tested typically provides each decoded pixel as an eight bit value (in Y,Cr,Cb component form or in color space converted R,G,B component form) at full pixel rate to an external tester. The external tester, which has a preloaded reference frame stored in memory for each decoded frame from the MPEG video decoder being tested, compares the pixel values from the MPEG video decoder under test with the pixel values of the preloaded references frames. Unfortunately, an external tester which performs this pixel by pixel comparison requires both an expensive high speed interface to the MPEG video decoder under test, and an expensive memory for storing multiple preloaded reference frames.
An alternative external tester employs a reference MPEG video decoder which is known to be good and which is exactly like the MPEG video decoder under test. During a test mode, the reference MPEG video decoder decodes the same test frames as the MPEG video decoder under test, and the external tester compares each pixel value from the MPEG video decoder under test to the corresponding pixel value from the reference MPEG video decoder. This testing arrangement eliminates the need for an expensive memory that stores multiple preloaded reference frames. However, the expensive high speed interface between the MPEG video decoder under test and the external tester, and a high speed pixel comparator, are required. Moreover, synchronization between the MPEG video decoder under test and the reference MPEG video decoder may be difficult to achieve.
The present invention solves one or more of the above noted problems.